The invention concerns an electromagnetic fan clutch for an internal combustion engine, said fan clutch comprising a drive shaft retained in a housing by spaced-apart first rolling bearings, said drive shaft being connected at one end to a fan rotor by a second rolling bearing that is configured as a three-ring bearing having a first ring, an intermediate ring and a third ring and rows of rolling elements arranged between said rings, the inner ring being firmly connected to the drive shaft, the outer ring being firmly connected to the fan rotor, the intermediate ring being adapted to be coupled to the drive shaft by a first electromagnetic clutch and the outer ring being adapted to be coupled to the drive shaft by a second electromagnetic clutch.
In automotive vehicles, cooling media are cooled by fans and the temperature of the cooling medium is used directly for controlling the electromagnetic fan clutch. With this electronically controlled clutch, it is guaranteed that the operating temperature of the engine is held at a constant high level, the engine performance is optimally utilized and the pollutant content in the exhaust gases is reduced.
An electromagnetic fan clutch of the pre-cited type is known, for example, from a prospectus of LINNIG ANTRIEBSTECHNIK GMBH 9/89. This prior art shows a drive shaft that is mounted in a housing through two spaced apart rolling bearings. This shaft is connected at one end to a belt pulley that is operatively connected to a crankshaft through a belt. At the other end, a fan rotor is connected to the drive shaft through a three-ring bearing. This electromagnetic fan clutch further comprises two electromagnetic clutches that can couple the outer ring and the intermediate ring of the three-ring bearing to the drive shaft. This assures that the fan can rotate at three different speeds of rotation.
A drawback of the prior art is that in certain states of operation, the rolling bearings of the three-ring bearing are practically at a standstill while the associated bearing rings rotate at a high speed. This results in a high degree of wear in the bearing of the fan rotor and leads to its premature failure. This high degree of wear of the bearing is further intensified by vibratory loads caused by the torsional vibrations of the crankshaft especially if the fan clutch is mounted directly on the crankshaft.
It is therefore an object of the invention to improve a rolling bearing for a fan rotor of the pre-cited type so that the rolling bearing withstands all conditions of operation of the electromagnetic fan clutch without any problem and therefore has a long operating life.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that at least in the second rolling bearing, different materials are paired with each other in regions of rolling contact. For the purposes of the invention, this means that the rolling elements and the associated bearing rings are made of different materials either over their entire cross-section or at least in a surface region. This results in change of the coefficient of friction at the points of contact between the rolling elements and the bearing rings which prevents damage to the bearing, particularly the damage that the rolling elements cause to their own raceways when they get pressed into them.
According to further features of the invention, the components participating in the rolling contact are provided with a wear-resistant hard coating. It is also sufficient if at least one of a row of rolling elements between the inner ring and the intermediate ring and a row of rolling elements between the intermediate ring and the outer ring comprises this wear-resistant hard coating. In one embodiment of the invention, this hard coating is provided on the raceway of the inner ring and the inner raceway of the intermediate ring, while in another embodiment, this wear-resistant hard coating is provided on the raceway of the outer ring and the outer raceway of the intermediate ring.
According to still another feature of the invention the material of the coating can be chromium (Cr), chromium nitride (CrN), titanium nitride (TiN), titanium carbonitride (TiCN) or tungsten carbide/carbon (WC/C). When paired with steel, these coatings manifest a high resistance to abrasion.
According to an additional feature of the invention, the chromium coating has a layer thickness of 0.01 to 8.0 xcexcm.
According to still another feature of the invention, the coating can be deposited by electrolytic or by PVD or CVD methods. Depending on the part to be coated, electrolytic deposition is performed using a stand or as a coating of mass-produced parts in drums or bells. The CVD method (Chemical Vapor Deposition) is based on the precipitation of solid matter out of the gaseous phase by chemical reactions. CVD technology utilizes the fact that volatile compounds react chemically upon input of heat and produce a corresponding precipitation. The term PVD covers coating methods through which metals and their alloys as well as chemical compounds such as oxides, nitrides, carbides are precipitated by an input of thermal or kinetic energy by particle bombardment under vacuum. PVD methods that are generally conducted under vacuum at temperatures between 500 and 550xc2x0 C., include vapor deposition under high vacuum, ion plating and cathodic sputtering.
Alternatively, the invention also achieves its objects by the fact that at least the second rolling bearing is filled with a solid lubricant which, if need be, is filled in a carrier substance which, according to a provision of the invention can be a lithium soap grease on a mineral oil base. In contrast to the first solution provided by the invention, the change in the coefficient of friction is not brought about by pairing different materials in rolling contact with each other but is exclusively guaranteed by the solid lubricant particles arranged between the friction surfaces.
A third solution offered by the invention for achieving its objects is a combination of the first two solutions.
According to a final feature of the invention, the second rolling bearing is a three-ring bearing with an inner and an outer two-row angular contact ball bearing in back-to-back arrangement, the pressure angle of the inner bearing being larger than the pressure angle of the outer bearing.